Aural tones are inaudible during engine start until N1 reaches what percent?

The correct answer is that aural tones become audible during engine start once N1 reaches 50%. This is crucial for pilots and crew to understand because aural tones serve as useful auditory alerts that indicate various status signals of the engine during its startup phase.

The N1 parameter refers to the speed of the low-pressure compressor in a turbofan engine, and it is an essential indicator of engine performance. During engine start, the engine undergoes several critical phases until it reaches a stabilized idle speed, marked by specific N1 values. The alerting system is designed to engage at a certain N1 threshold to ensure that the crew is aware of engine status while avoiding unnecessary noise during earlier stages of startup when adjustments are being made and stabilization is still in progress.

Recognizing the right point at which aural tones become available allows the crew to monitor the engine's performance effectively and respond to any alerts with the necessitated attention that some scenarios may demand.